This invention relates to a method for making a double diffused vertical NPN transistor with a self aligned emitter to emitter-contact, and more particularly relates to a method employing a second doping step through the emitter-contact-hole for locally deepening the base region under the emitter.
For making bipolar transistors it is known to form a self aligned metal contact to an emitter by diffusing impurities through a hole in an overlying glass layer and filling the hole with contact metal. This has the advantages of self alignment between the emitter and the emitter-contact that makes it possible to reduce the size of the emitter. It also prevents metal contacts subsequently introduced via the hole from doping the silicon surface and making it more positive.
It is often desired to make integrated circuit components in an epitaxial layer of minimum thickness so as to require short doped regions that must run through the epitaxial layer from the surface to the underlying substrate and to buried layers.
By reducing the length of such doped regions, the amount of surface area required by such through-epi-doped regions is reduced. Double-diffused NPN transistors may be made in an N-type epitaxial layer of a minimum thickness that is generally limited by the depth of the base region.
It is an object of this invention to provide a method, for making a double diffused NPN transistor capable of having a small emitter and a thin base region, that is compatible with methods for also making integrated circuits including field effect transistors.